Field of the Invention
The present invention relates to a fuel cell cogeneration system including a fuel cell module for generating electric power by electrochemical reactions of a fuel gas and an oxygen-containing gas, a method of starting operation of the fuel cell cogeneration system, and a method of operating the fuel cell cogeneration system.
Description of the Related Art
In general, a solid oxide fuel cell (SOFC) employs a solid electrolyte. The solid electrolyte is an oxide ion conductor such as stabilized zirconia. The solid electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (hereinafter also referred to as an MEA). The electrolyte electrode assembly is sandwiched between separators (bipolar plates). In use of such fuel cells, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack, which is applied to a fuel cell module.
The SOFC is operated at relatively high temperature. The exhaust gas including fuel gas and oxygen-containing gas consumed in the power generation reaction has high temperature as well. Therefore, it is desired to achieve effective utilization of the exhaust gas.
For example, systems having a heat exchanger for performing heat exchange between the exhaust gas from the SOFC and water, and a hot water tank storing the water have been adopted. The water in the hot water tank is heated by the heat exchange to produce hot water having a predetermined temperature. The hot water is supplied to a hot water system or a heating system for home use. That is, the systems are fuel cell cogeneration systems.
In the fuel cell, it takes considerable time to warm up the fuel cell to a desired operational temperature after starting operation of the fuel cell. For example, in the case of starting operation of the fuel cell at low temperature such as a temperature below the freezing point, etc., or in the case where a SOFC, which has a high operating temperature, is used, the start-up time required for starting operation of the fuel cell is significantly long.
In this regard, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2006-179198 (hereinafter referred to as the conventional technique) is known. In this conventional technique, at the time of starting operation, a coolant flows through a bypass channel. Then, power generation of a fuel cell stack is started. Further, operation of a heating apparatus is started to thereby heat the coolant circulating to bypass the fuel cell. When the temperature of the coolant reaches a predetermined temperature or more, the flow of the coolant is switched to a channel passing through the fuel cell stack.
According to the disclosure, in the structure, it becomes possible to prevent the cold coolant from flowing into the fuel cell stack, and self-heating of the fuel cell stack by power generation is facilitated. Further, according to the disclosure, the bypass channel and the coolant inside the bypass channel have smaller heat capacity in comparison with the fuel cell stack itself and the coolant inside the fuel cell stack, and therefore heating of the bypass channel and the coolant inside the bypass channel can be performed in a relatively short period of time even if a heating apparatus having a small heat capacity is used.